Inertial force sensors are used in numerous consumer products. For example, various occupant protection systems in automobiles utilize such sensors to trigger actions designed to protect the vehicle occupant when the automobile is involved in a collision or the system senses that a collision is imminent. Such systems are used to actuate air bags in the event of a collision and to pretension seatbelts when sensors indicate that a collision is about to occur.
These sensors typically utilize a weight and spring arrangement in which a moveable weight is fixed to a stationary component by a spring. When the apparatus is accelerated or decelerated, the weight moves relative to the stationary component. Such sensors typically include a transducer that converts the position of the weight relative to the fixed component, or the rate of change in that position, into an electrical signal representing the displacement or rate of change of the position, respectively.
The transducers rely on a variety of techniques to convert the motion of the mass into an electrical signal. In the simplest schemes, the mass is mounted on a cantilever that provides the spring function. A contact on the cantilever makes an electrical connection with a contact on the stationary component when the mass moves a predetermined distance. Systems in which the cantilever includes a piezoelectric element that generates a current in response to the bending of the element are also known.
The accuracy with which the force on the mass can be measured depends on the reproducibility of the spring constant and mass from device to device. In addition, the accuracy depends on the sensitivity of the transducer. Low cost sensors are often fabricated using micro-machining techniques. If a simple threshold measurement is all that is required, the reproducibility limitations can be easily met with such techniques. Such sensors are sufficient for use in triggering airbag deployment in an automobile. However, if a more accurate analog measurement is needed, the device may need to be individually calibrated leading to increased cost.
In addition, the amount of motion that must be sensed in these miniature devices requires a transducer that has high sensitivity, particularly if an analog measurement of the displacement is needed. The cost of such transducers can limit the applications in which such analog sensors can be utilized.
In addition, these sensors must often operate in a hostile environment in which the temperature varies over a large range and in which dirt and other contamination can buildup leading to device failure. Hence, the devices must typically be sealed in a manner that keeps out the contamination while still allowing the sensor to function properly.
Furthermore, these sensors typically require one transducer per degree of freedom. Hence, if motion in multiple dimensions is to be tracked, multiple transducers are required. The additional transducers increase the cost of the sensor.
Finally, these sensors are subject to oscillations resulting from resonances in the spring-mass system. Hence, some means for damping such oscillation is often required. The damping mechanism must not interfere with the transducer mechanism, and hence, there are constraints on the damping mechanism that further increase the cost of the sensor.